Monolithically integrated light-actived thyristor and method

ABSTRACT

A monolithically integrated light-activated thyristor in an n-p-n-p-n-p sequence consists of a four-layered thyristor structure and an embedded back-biased PN junction structure as a turn-off switching diode. The turn-off switching diode is formed through structured doping processes and/or depositions on a single semiconductor wafer so that it is integrated monolithically without any external device or semiconducter materials. The thyristor can be switching on and off optically by two discrete light beams illuminated on separated openings of electrodes on the top surface of a semiconductor body. The carrier injection of the turning on process is achieved by illuminating the bulk of the thyristor with a high level light through the first aperture over the cathode to create high density charge carriers serving as the gate current injection and to electrically short the emitter and drift layer. The switching off of the thyristor is achieved by shorting the base layer and the cathode layer by illuminating the embedded back-biased PN junction of the TURN-OFF switching diode. The patterned doping profile and the interconnect between the emitter and the base region of the light activated thyristor makes possible a monolithic and/or plantar integrated fabrication of the semiconductor switching device on a single semiconductor wafer via the standard semiconductor fabrication process.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to and is a divisional of U.S.application Ser. No. 12/5889,580 filed Sep. 24, 2010 which is adivisional of U.S. application Ser. No. 12/507,100 filed Jul. 22, 2009which is a divisional of Ser. No. 11/371,940 filed on Mar. 10, 2006.

FIELD OF INVENTION

The present invention relates generally to the field of powerelectronics and, more particularly, to an optically turn-on and turn-offpower thyristor.

BACKGROUND OF THE INVENTION

Compared to other semiconductor power switching devices like MOSFETs andIGBTs, the thyristor is generally known for its ability to sustain largecurrent and its ability to be switched at high voltage. With a lowon-state voltage drop for a given current density, a thyristor providesone of the lowest power dissipations among power semiconductor devices.A thyristor typically has four basic semiconductor layers, the emitter,base, drift and anode layers, respectively, with an alternative dopingprofile to form three junctions. Adjacent layers are oppositely dopedwith high doping on two outer layers and light doping on inner layers.When a voltage is applied between the anode electrode and cathodeelectrode, at least one junction is reverse biased to sustain a majorityof the applied voltage, and the voltage is held mainly across thelightly doped drift layer until its breakdown.

A thyristor can be viewed as a pair of back-to-back coupled bipolarjunction transistors. The lightly doped inner regions act as the base oftwo transistors. When a thyristor is under high voltage withoutprotection, the leakage current across the lightly doped regions servesas the base current of the transistors and is amplified. When there areenough earners inside the inner layers, the device is turned on. Thefirst method of turning on a thyristor is, in fact, to apply a voltagehigher than its blocking value. When the applied voltage is high enough,the thyristor is turned on through the current gain of the leakagecurrent. However, the turn-on voltage is not precisely controllable andhigh voltages sometimes induce damage inside the device throughbreakdowns. In practice, the leakage current diverting structures likethe cathode short and the anode short are added in the design to enhancethe voltage holding capability.

For more controllable turn-on switching, the carriers are injected froma gate electrode on one of the inner layers. Usually, the gate currentis only injected over a portion of the base layer, so the conductingcurrent of a thyristor is not fully spread over the whole layerinitially. The thyristor will not be fully turned on until carriersspread across the layer by lateral diffusion. The time of carrierspreading depends on the lateral diffusion velocity, which limits theturn-on time. One way to circumvent the slow turn-on time is to injectthe gate current over a large area. This may, in practice be implementedwith inter-digitization of the gate electrodes and reduces the activecathode area for supporting high current.

An alternative approach for turning on a thyristor is to generatecarriers locally inside the inner junctions through absorption of light.There have been several attempts in the prior art to use a photonic gateover the cathode area which permits light to pass through.Photo-generated carriers acting as the gate current injection in thebase region start the turn-on process. With appropriate selection of thelight wavelength, the depth of the light absorption across the devicecan be varied to fit different junction depths. Furthermore, a highlevel of illumination across the whole thyristor structure caninstantaneously generate a high density of carriers across the wholedevice. The high density of carriers collapses the junction voltage andgenerates the current flow instantly without much delay from carriersbeing transported through the thyristor drift layer and the lateraldiffusion from the gate electrode area to the main cathode area.Therefore, a light controlled thyristor has the advantages of shorterturn-on delay time and shorter turn-on time.

In practice, thyristors are employed in power circuits with highvoltages and large currents. The trigger circuit to switch a thyristorthrough the gate electrode is difficult to isolate from high voltages.Instead of triggering through an electrical gate current, a high degreeof electrical isolation between the power and trigger circuit may beachieved by switching with light through optical wave guide-like fibers.

Power MOSFETs and IGBTs will be switched off if the gates are turnedoff. Due to the self-sustaining effect of a thyristor, the currentconduction of a thyristor will continue even after the gate is turnedoff. A thyristor does not need to maintain the gate injection like otherpower semiconductor devices. However, the gate of a thyristor losescontrol after the thyristor is switched on. To actively switch athyristor from its on-state to the forward-blocking state can only beaccomplished by reducing its current below a threshold or by reversal ofthe anode voltage. In an AC circuit, a thyristor is switched on and offin a cycle while the polarity of the voltage is alternative across theanode and cathode electrodes. However, it is not practical to switchpolarity to reverse the anode voltage of a power device in manyapplications. Typically, the thyristor current is drained through areverse gate current during turn-off.

In early attempts, the Gate Turn-Off thyristor (GTO) utilized anexternal control circuit to reverse the gate current and the MOSControlled Thyristor (MCT) incorporates parallel MOSFETs to createemitter shorts. The external control circuit of a GTO diverts thecurrent through the gate electrode and needs to carry a similar amountof current as a thyristor in order to switch off the thyristor. Thediverted current is much larger than the turn-on gate current and thisincreases the difficulty of the control circuit design. Typically, theexternal current-diverting circuit of a GTO is much bigger in order toaccommodate large thyristor current and there is basically no electricalisolation between the power arid trigger circuits. On the other hand, aMCT uses MOSFETs to short the emitter and the base of a thyristor. Likethe case of the external turn-off circuit of a GTO, MOSFETs in a MCTalso need to take on the majority of the thyristor current. However, thecurrent carrying capability of a MOSFET is limited by its surfacechannel and is much smaller than the bulk of a thyristor. Therefore,massively parallel MOSFET unit cells are integrated in order to carrylarge current. The MOSFETs occupy large real estates and limit the mainconduction area of a MCT. Hence, MCTs have not been widely used forpractical applications.

The limitation of the electronic switching-off of a thyristor is eitherdue to the current carrying capability of MOSFETs to create emittershorts or the limited current and speed of external current-divertingcircuits. In addition, the electronic on-activation of a thyristorsuffers slow turn-on and long delay due to the limited speed of carriertransport. A new control technique is needed to improve these shortfalls.

In earlier attempts, a photonic gate was employed in the lightcontrolled thyristor to permit light. The illumination of light throughthe photonic gate generates carriers in the base region as the gatecurrent injection to turn on the device. However, an external circuit isstill employed for turning off a light controlled thyristor to divertthe conducting current as a GTO. Various attempts with alternativeelectronic switching-off schemes also suffer similar shortfalls. Hence,the light controlled thyristor did not solve the whole problem thatlimits thyristors in many applications.

To improve the turn-off limitation of the light controlled thyristor, aphotonic controllable switching structure was introduced on a thyristor.In O. S. F. Zucker et. al. (U.S. Pat. No. 6,218,682 B1, Apr. 17, 2001),the optically-activated thyristor adds an external shorting structure ontop of a light activated thyristor. The shorting structure iselectrically and mechanically bonded across the emitter and base regionof a thyristor. The added shorting structure comprises a PN junction andhas an optical aperture for introducing light. Furthermore, an apertureover the emitter region for permitting light is introduced to betterutilize the wafer surface area for current conduction instead ofseparated photonic gates. Therefore, a high level of light illuminationmay be introduced through the aperture to generate high density carriersin the bulk of the thyristor to direct short the whole device for fastswitching on. During the conducting state of the thyristor, the shortingPN junction is open and under the back bias. When light is introducedonto the shorting structure, the photo-generated carriers collapse thevoltage and electrically short the cathode and the base of the thyristorto create emitter shorts. The illuminated shorting structure diverts theconducting current to bypass the emitter and then turns off thethyristor. However, the main thyristor structure and the shortingstructure are fabricated separately on different semiconductor wafers.The wafer with the shorting structure is then diced and externallybonded on the main thyristor structure. In addition to the alignment ofthe optical fibers to apertures, additional alignment and bonding of theshorting structure to the main thyristor structure in the back endprocessing increase the complexity and cost of the fabrication.

Accordingly, there remains a need for an improved light activatedthyristor. There remains a further need for a thyristor that is compactand monolithically integrated, so that the complexity and cost offabrication may be greatly reduced to fit practical applications.

SUMMARY OF THE INVENTION

According to the present invention, a monolithic, high power thyristoron a semiconductor wafer is provided. The thyristor may incorporatephotonic switching control. The monolithically integrated lightactivated thyristor comprises four alternatively doped layers of a basicthyristor structure, an emitter, a base, a drift and an anode layer,respectively, and additional two oppositely doped zones monolithicallyintegrated on top of the emitter layer. The added two oppositely dopedzones may be formed of the same or different semiconductor materials.According to one embodiment, the adjacent layers and zones of themonolithically integrated light activated thyristor are of the oppositedoping and form an n-p-n-p-n-p structure.

During the on-state of the thyristor, the two oppositely doped zones onthe top form a back-biased PN junction acting as a switching diode. Thezone adjacent to the emitter layer with the opposite doping iselectrically shorted with the emitter layer on the top surface to thecathode and the other is electrically connected to the base layerthrough interconnect over an insulating layer. Two apertures forpermitting light control are formed by openings of the cathode electrodeover the emitter region and of the floating gate over the junction ofthe switching diode. The construction of the monolithically integratedlight activated thyristor is achieved through the processes of spatiallydoping and/or depositions on a single semiconductor wafer, without anyexternal attachment of addition semiconductor devices.

The overall device consists of multiple cells of functionally identicalunits. In each unit device, there are two sets of optical apertures onthe top surface: one is on the cathode acting as a gate for the turn-onprocess; the other is over the back-biased junction of the embeddedswitching diode acting as a switch to turn off the thyristor.

According to one embodiment, a method of activating the thyristorincorporates applying a voltage across the top emitter layer and thebottom anode layer of the semiconductor device. The turn-on process isachieved by illuminating light through the cathode aperture andphoto-generating carriers on the base layer and the drift layer actingas the gate current injection. Furthermore, a high level illuminationgenerates a high density of charged carriers to collapse the voltageacross the blocking junction and turns on the whole device throughdirect current flow and/or lateral carrier diffusion. The device is kepton through the thyristor regenerative action even when the illuminationis stopped.

According to another embodiments, an embedded back-biased switchingdiode structure incorporated on top of the emitter layer seizing as aturn-off switch between the cathode electrode and the base layer. Whenilluminating the aperture over the junction of the embedded switchingdiode, the photo-generated carriers collapse the back-biased junctionand electrically short the base layer and the cathode. The current thenbypasses the emitter layer to stop the injection through theemitter-base junction. Hence, the current diversion stops the thyristorregeneration effect and turns off the thyristor when the current isreduced below the holding level.

The light controlled turn-on and turn-off processes allow the electricalisolation of the trigger circuit from high voltages and have acapability of remote control through a light guiding scheme such as theutilization of optical fiber. Moreover, the present invention allows theturn-off switch diode structure to be monolithically integrated into thesame semiconductor wafer during fabrication. The embedded switchingdiode can be incorporated in the fabrication process, without anybonding or integration of the external switching device in the back-endprocess. The present invention accordingly provides a better integrationof power semiconductor devices in many circuits and reduces thecomplexity and cost of fabrication.

The above described features and advantages of the present inventionwill be more fully understood with reference to the detaileddescription, drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows, in cross section, a unit cell of an embodiment comprisinga four layer thyristor structure, an embedded turn-off switching diode,and two separated optical apertures for turning on and off control withlight beams, according to an embodiment of the present invention.

FIG. 2 shows, in cross section, a unit cell of the embodiment in anon-planar construction. The layer structure is formed through multiplesteps of spatial doping processes and/or depositions and etching,according to an embodiment of the present invention.

FIGS. 3A and 3B show circuit representations according to an embodimentof the present invention.

FIGS. 4-17 show the various steps of a fabrication process, according toone example of an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a high power, monolithically integratedthyristor-based device with a switching diode structure thatincorporates optical on and off control.

The semiconductor devices described herein are based on the use of lightto actively switch on and/or off, and are referred to by the genericname “light controlled thyristors” (LCTs) or “optically controlledthyristors” (OCTs). Optical activations involve illuminating asemiconductor device with light to create electron-hole pairs at thesite of absorption and do not require the injection of carriers throughcarrier transport. Hence, the optical activation of the device may befaster than the injection, which is limited by drift velocity andlateral diffusion of carriers.

In addition to switching on, the present invention provides the abilityto actively switch off a power semiconductor device with light. Theunderlying principle for switching off a thyristor is to create anelectrical short across the emitter-base injection junction. The emittershort is accomplished by using light to illuminate the back-biasedjunction of the embedded-switching diode structure connecting the baselayer and the cathode electrode. The selection of the optical wavelengthof the activating light results in an ability to control the absorptionlength, and hence the volume and depth of the semiconductor materialactivated by the light. In addition, the carrier concentration in theswitching junction may also be controlled by the amount of light.

The illuminating light may be generated by a separate circuit far fromthe power circuit. Light may be introduced into the device through anoptical wave-guide, such as an optical fiber. The electrical isolationof the trigger circuit from the main power circuit may be realized byphotonic switching on and off. Furthermore, the monolithic integrationof switching-off diodes, according to an embodiment of the presentinvention, allows an optically controlled thyristor to be fabricated ona single semiconductor wafer and reduces the complexity and cost offabrication.

According to an embodiment, an array of functionally identical unitcells is introduced onto a semiconductor wafer. The total number of unitcells may be varied to fit the requirement of applications withdifferent layouts. An example of fabrication steps of a unit cell of thepreferred embodiment is illustrated in FIGS. 4-17.

Referring to FIG. 4, beginning with a portion of a lightly doped n-typesemiconductor wafer 200, a n-type drift layer 204 is sandwiched by twop-type layers (the base layer 204 on the top and an anode layer 206 onthe bottom). The two p-type layers and subsequent layers may be formedby diffusion, ion implantation, epitaxial growth, or any otherappropriate technique. The p-type anode layer may be heavily doped andthe p-type base layer may be lightly doped.

An additional n-type-emitter layer is then introduced onto the topp-type base layer 202, as shown in FIGS. 5 and 6. The profile of theemitter layer is formed by coating the top surface of the p-type baselayer 202 with a masking layer of suitable choice for photo-resistance.The masking layer initially covers the whole surface, and portions areremoved by the photo-lithographic technique to form a masking pattern230 as shown in FIG. 5. The covered areas of the masking pattern are fora plurality of cathode shorts and gate contacts. The top emitter layer208 is then introduced by diffusion or ion implantation on the openingas shown in FIG. 6. The resulting semiconductor body 200 has the basicn-p-n-p structure of a thyristor with a plurality of cathode shorts 207and gate contacts 209 on the top surface. Various modifications may bemade in terms of doping profile and layer thickness to optimize thedevice electronic properties such as maximum forward and/or reserveblocking voltage, switching characteristics, and other properties.

A second masking pattern 232, as shown in FIG. 7, covers portions of thetop surface of the emitter layer 208 and has an opening for theintroduction of an additional p-type doping zone 210 by diffusion orion-implantation as shown in FIG. 8. Subsequently, a third maskingpattern 234 as shown in FIG. 9 may be introduced on top of the p-typezone to add a n-type doping zone 212 as shown in FIG. 10. The last twooppositely doped zones, 210 and 212, form a PN junction and that may actas a switching diode 211 for the turn-off process.

An insulating layer 214 is deposited to cover the top surface of thesemiconductor body 200 as shown in FIG. 11. The anode electrode 216 maybe added by a contact metallization on the bottom surface as shown inFIG. 12. For the top metallization, the insulating layer 214 on the topsurface may be first masked with the pattern 236 as shown in FIG. 13.The opening portions of the insulating layer are removed by etching. Thetop is then metallized to form the cathode electrode 224, the gateelectrode 226 and the cathode (n-type) end contact 228 of the switchingdiode as shown, for example, in the steps illustrated in FIGS. 14 and15. The n-type emitter layer 208 and the anode (p-type) zone 210 of theswitching diode are electrically shorted by the cathode 224. Inaddition, a plurality of the cathode-shorts 207 distributed on thecathode electrode 224 of a device suppress the gain of the parasitic topNPN transistor to improve the forward blocking voltage.

The insulating layer is transparent to light and two optical apertures218 and 220 in each device unit resulted from the masked and un-etchedareas on the top surface. The optical aperture 218 is an opening in thecathode electrode over the emitter layer for permitting turn-on light.The optical aperture 220 is over the junction area of the embeddedswitching diode 211.

The gate 226 and the diode cathode contact 228 are linked by firstmasking the top surface with the pattern shown in FIG. 16 andinterconnecting with subsequent metallization over the insulating layeras shown in FIG. 17. The floating gate comprised of the interconnect222, the gate 226 and the diode cathode contact 228 is kept floatingduring operation and diverts the thyristor current when the embeddedswitching diode is shorted by light.

Another embodiment is shown in FIG. 2 with a non-planar construction.The layers of the embedded switch-off diode-structure may be depositedepitaxially or in poly-crystallized form, or wafer bonded on top of theemitter layer. The wafer bonding may involve bonding wafers togetherhaving the same and/or different materials. The non-planar constructionmay be fabricated by following a mesa etching processes. The top surfacemay also be smoothed through the standard planarization process or anyother process. The deposited layers may be different semiconductormaterial from the underlying layers of the basic thyristor structure forselection of optical wavelength.

Having described planar and non-planar embodiments and associatedmethods of manufacturing, it will be understood by those having ordinaryskill in the art that changes may be made to those embodiments withoutdeparting from the spirit and scope of the present invention.

In operation, the monolithically integrated light activated thyristor200 in FIG. 17 may be connected to a circuit through the anode electrode216 and the cathode electrode 224, while the floating gate electrode 222is kept floating. For a forward blocking operation, the anode electrode216 may be forward biased relative to the cathode electrode 224. Underhigh voltage, a small forward leakage current may pass through thedevice and multiple cathode-short regions 207 in a device provide aprotection against premature turn-on through the leakage current gain.In addition, a voltage holding capability exists and may be enhancedthrough manipulation of resistivity across the embedded switching diode211. Illuminating light through the aperture 220 also may create cathodeshorts through the change in resistivity of the embedded diode.

In the turn-on process, a light pulse is initially introduced throughthe turn-on aperture 218, preferably via an optical fiber, to illuminatethe main body of the thyristor 200. The optical pulse generates a denseconcentration of electrons and holes through absorption across thedevice. The photo-generated carriers, acting as the base currentinjection from the gate in the conventional thyristor, collapse thedepletion region across the p-type base region 202 and the n-type driftregion 204. Shorting the n-type emitter layer 208 and the n-type driftlayer 204 results in the forward conduction state.

The thyristor 200 stays on through regenerative action even after thelight is turned off. The turn-on process, utilizing light activation, isrelatively fast compared to electronic turn-on thyristors. To generatephoto-carriers, the photon energy of the light pulse should be above theenergy band gap of the semiconductor material in use. The penetrationdepth of the light in the device may be adjusted by varying thewavelength of the activation light such that the illuminating lightreaches through the p-type base layer 202 and into the n-type driftlayer 204.

In an embodiment of the monolithically integrated light activatedthyristor, the embedded switching diode 211 is comprised of the p-typeregion 210 and the n-type region 212. The cathode electrode 224 laysover both the n-type emitter 208 and the anode (p-type) junction side210 of the switching diode 211. The p-type base layer 202 iselectrically connected to the cathode (n-type) junction side 212 of theswitching diode 211 through the floating gate 222 and is insulted with adielectric layer from the p-type region 210 of the switching diode 211and the n-type emitter region 208. While the thyristor is under forwardbias, the PN junction of the embedded switching diode 211 isback-biased, with high resistance.

To turn off the thyristor, the embedded switching diode 211 isilluminated with a through the turn-off aperture 220. This results inthe p-type base layer 202 being electrically shorted with the cathodeelectrode 224. The emitter short results in current bypassing the n-typeemitter region 208. Subsequently, it terminates the self-injection intothe p-type base layer 202 and turns of thyristor 200.

In order to avoid unwanted carrier injection onto the p-type base layer202 during the turn-off process, the illuminated light should have ashorter absorption depth compared to the turn-on light such that it doesnot reach the depth of the p-type base layer 202. As mentioned above, anappropriate wavelength may be selected to fit the requirement. Inaddition, the deposition Of different semiconductor materials also mayaccommodate different absorption wavelengths such that the thyristorstructure is transparent to the turn-off light. The resistance of theswitching diode 211 may be controlled by the amount of light introducedthrough the aperture 220. To prevent the high dV/dt turn-on during theforward blocking state, a low level of light may be introduced onto theswitching diode 211 to accommodate the rapid change of the anode voltagein a circuit. In addition to the cathode short 207, a low level ofilluminating light also lowers the resistance of the switching diode 211to pass through the induced current flow due to the built-in capacitanceof the thyristor 200. The low level light may be turned off during theturn-on process so as to maintain the on-state voltage drop across then-type emitter 208 and the p-type base 202.

In summary, according to an embodiment of the present invention, duringthe forward blocking state of the thyristor 200 operated in a circuit,the resistance of the embedded switching diode 211 is modulated by lowlevel light so that a resistive emitter-short is created to increase thedV/dt hold-off capability. The elimination of the low level lightthrough the aperture 200 recovers the high resistive state of theswitching diode 211 for the thyristor on state. To turn on thethyristor, a high level of illumination through the aperture 218generates photo-carriers across the main body of the thyristor 200 andturn-on the device. To turn off the activated thyristor 200, a highlevel light is introduced onto the embedded switching diode 211 throughaperture 220 to create an electrical short between the p-type base layerand the cathode 224 to terminate the self-injection within the thyristor200 and turn off the thyristor 200. Once the thyristor 200 is turnedoff, the illumination of a low level light over the embedded switchingdiode 211 may be resumed to enhance the dV/dt hold-off capability inaddition to the cathode short. FIGS. 3A and 3B depict potentialschematics of this design according to an embodiment of the presentinvention. Referring to FIG. 3A, three devices are illustratively showncoupled together, respectively 300, 305 and 310. A N type region ofdevice 300 is illustratively shown as coupled to respective P typeregions of devices 305 and 310. This node has been described herein as afloating gate node. A P type region of device 300 is shown as coupled tothe cathode and one of the N type regions of device 310. The other Ntype region of device 310 is shown coupled to the N type region ofdevice 305. The other P type region of device 305 is coupled to theanode. Referring to FIG. 3B, devices 320, 325 and 330 respectively showelectrical representations of devices 300, 305 and 310.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

In particular, it be apparent that while particular semiconductorstructures have been illustrated and while particular processing stepshave been shown, numerous variations are possible and contemplated bythe applicants. For example, it will be understood that the circuitshown in FIGS. 3A and 3B may be physically realized in an integratedsemiconductor structure in many different ways. The Figures depict oneway of implementing an embodiment of the invention in a semiconductorstructure. However it will be understood that the semiconductor layeringscheme may be changes, and the junction locations and profile, insulatorlayers and contacts may be changed for any reason. In addition, whilesilicon has generally be described, it will be understood that any othertype of semiconductor structure may be implemented.

1. A method of switching an optically controlled thyristor structure having an anode, a cathode, a base layer and a drift layer, comprising: illuminating the thyristor base layer and the thyristor drift layer with light through at least one aperture formed through thyristor cathode electrode to generate photo-carriers in a light penetrated region to turn on the thyristor. 